The science of photonics includes the generation, emission, transmission, modulation, signal processing, switching, amplification, detection and sensing of light. Whilst a proportion of the population are aware that photonics has enabled the explosion of bandwidth and services accessible to them within the field of telecommunications and the Internet, and a much larger proportion are probably aware of solar cells and their ability to generate electricity from the sun, everyone is aware of lights and how they impact their daily life allowing essentially any activity other than sleeping outside the hours the sun is over the horizon.
Solid State Lighting: The ability to generate light with electricity by Sir Humphrey Davy 200 years ago sparked a century of development by the likes of Thomas Edison, Joseph Swan, Sandor Just (tungsten filaments), and Irving Langmuir (inert gas instead of vacuum) leading to the establishment 100 years ago of tungsten filament lamps, which as the dominant light source have fundamentally shifted how people live, work, play. However, their efficiency is woefully low, being only 2.1% for a 60 W incandescent light and only 3.5% for a quartz halogen. Accordingly there is a massive worldwide campaign to replace incandescent lights by compact fluorescent lights (CFL) with an efficiency of 22% thereby reducing energy consumption significantly. However, CFLs are not a panacea as issues exist including lifetime, health and safety issues from mercury content, UV emissions, health issues for some individuals, radio interference, low luminance, dimming, and recycling due to the phosphor and mercury. Further CFL efficiency drops with increasing/decreasing temperature from room temperature and non-operation is typical below freezing.
However, a monochromatic solid state light source within the visible wavelength range can achieve in principle an efficiency approaching 100%. Additionally solid state light sources should also reduce consumption of precious metals, reduce recycling, remove health/safety issues and permit operation at all temperatures. Beneficially solid state light sources by virtue of their small size, low weight, and low voltage operation can also be employed in a wide range of situations where incandescent or CFL lights cannot. At present niche applications such as holiday decorations in conjunction with indicator lighting in panels, backlighting in LCD displays etc mean that solid state lighting sales today account for only approximately 2% of the current lighting market and will grow to only approximately 3% in 2011.
Despite this solid state lighting is a massive market which according to NextGen Research (“LEDs and Laser Diodes: Solid State Lighting Applications, Technologies, and Market Opportunities”, February 2009, http://www.nextgenresearch.com/research/1001995-LEDs_and_Laseτ_Diodes) forecasts the overall solid-state lighting (SSL) market will achieve worldwide revenues topping approximately $22 billion in 2011 and $33 billion by 2013. The illumination segment of the LED market will see compound annual growth rate (CAGR) of nearly 22% in the 2009-2013 timeframe. The display portion of the market also will achieve a five-year CAGR of over 14% as cities worldwide shift their streetlights to these more energy-efficient and ecologically friendly solutions. However, the majority of this growth will be generated from niche lighting applications including architectural, task lighting, medical and off-grid lighting applications rather than the residential lighting market according to The Strategy Analytics (“LED Device and Material Market Trends”, June 2009, http://www.strategyanalytics.com/default.aspx?mod=ReportAbstractViewer&a0=4788).
As such the majority of the lighting market, which in 2011 will be approximately 97% representing approximately $700 billion in revenue, remains inaccessible despite the considerable research effort and investment expended to date. Hence, for solid state lighting applications the ultimate goal is a high efficiency white LED allowing access to this vast currently inaccessible market.
However, prior art LED structures whilst offering a fairly broad wavelength range operate at relatively low efficiencies and typically three LED devices are required to even cover a substantial portion of the wavelength range to which the human eye is responsive, the so-called visible wavelength range, which is 380 nm to 750 nm. As such red, green and blue centered LED devices are typically used to create the impression of white, of which blue LEDs were the last to be developed based upon InGaN structures. At present the challenges in realizing suitable LED technologies and devices for lighting applications include their relatively low internal quantum efficiency, low light extraction efficiencies realized, and the relatively high device fabrication costs. Blue LEDs also form the basis of many “white” LEDs today that employ a phosphor-conversion scheme, but which sets the ultimate quantum efficiency of these “white” LEDs to below 65%.
The increase in efficiency of LEDs by the introduction of quantum confined structures, such as quantum wells, multi-quantum wells etc also results in a narrowing of the optical emission from the source. Accordingly, with prior art solutions increasing the efficiency of the sources require that number of sources required to “blend” together for the desired white light also increases thereby impacting the financial aspects of employing solid state lighting. It is in this regard that the high luminescence efficiencies, low fabrication costs, and processibility of semiconductor nanostructures have made them promising candidates for future lighting devices and the subject of considerable research and development. These semiconductor nanostructures include quantum dots and nanowires.
Solar Cells: The ability to generate electricity from sunlight has been touted as one of the means along with water and wind for reducing society's dependency upon fossil fuels and an alternative to increasing the number of nuclear power stations. In many parts of the world access to dependable wind or water as a means of generating power does not exist, added to which such installations tend to be geared to generating significant power to support industry and/or urban environments. However, solar power may be deployed essentially anywhere and can augment as well as replace conventional means of generating electricity. With the sun providing roughly 200 Wm−2 the global energy consumption in 2005 was 0.014% of the solar energy reaching the earth, and projections for 2100 are 0.051%. Hence solar power should be able to provide sufficient energy for most of our needs. In 2005 solar cells accounted for only 0.0037% of energy consumption globally. By 2050 that is expected to increase to 30.7% even whilst global consumption increases nearly 200% in the same time frame. Accordingly projections for this still as yet emerging market are for massive revenue growth long-term although short-term factors such as oil prices, Government policies make projections short-term difficult. As a benchmark roughly 0.01% of global energy production, based upon 40% CAGR from 2005, represented 2008 revenues of $37.1 billion. Solar cells in 2008 averaged $3/W (http://www.solarbuzz.com/Marketbuzz2009-intro.htm).
By far, the most prevalent material for solar cells is bulk (i.e. wafer) silicon, be it monocrystalline, polycrystalline, or amorphous with efficiencies from 6% to 14-19%. Existing commercial alternatives include thin film cadmium telluride, copper indium selenide (14%), and copper indium gallium selenide (19%) although current manufacturing costs are significantly higher than silicon. Much research and development is focused to multi-junction cells, for example consisting of GaAs, Ge, and GaInP which offer efficiencies from approximately 30% to over 40% but at present such cells cost about one hundred times as much as an 8% efficient amorphous silicon cell in mass production whilst only delivering about four times the electrical power. As such these multi-junction cells have tended to be deployed in space applications. Multi-junction cells partition the spectrum into bands such that a different semiconductor absorbs each band, an approach similar to that outlined above to provide a “white” light source from multiple LEDs.
Increasing the efficiency of semiconductor photodetectors without introducing avalanche multiplication by the introduction of quantum confined structures, such as quantum wells, multi-quantum wells etc similarly results in a narrowing of the optical absorption as it does a narrowing of emission from optical emitters. Accordingly, with prior art solutions increasing the efficiency of the photodetectors requires that number of photodetectors required to “blend” together to cover the full solar spectrum also increases. As such high absorption efficiencies coupled with potentially low fabrication costs and processibility of semiconductor nanostructures have made them promising candidates for future solar cell devices and the subject of considerable research and development. These semiconductor nanostructures include quantum dots and nanowires.
InGaN, Nanowires and Quantum Dots: With the recent discovery that the band gap of indium nitride (InN) was approximately 0.7-0.8 eV (1750 nm), see for example J. Yu et al in “Unusual Properties of the Fundamental Band Gap of InN,” (Appl. Phys. Lett., Vol. 80, pp. 4741, 2002) and T. Matsuoka in “Optical Bandgap Energy of Wurtzite InN” (Appl. Phys. Lett., Vol. 81, pp. 1246 (2002)), is combined with the fact that the bandgap of GaN is at approximately 3.3 eV (370 nm), and these represent the extremes of the quaternary alloy InGaN then the absorption of this alloy can be continuously tuned from ˜0.7 eV to 3.3 eV, thereby matching almost perfectly to the solar spectrum. As such InGaN has also emerged as a promising material for future high-efficiency full solar spectrum solar cells, E. Trybus et al “InN: A Material with Photovoltaic Promise and Challenges” (J. Crystal Growth, Vol. 288, No. 2, pp. 218-224, 2006) as well as for light sources (LEDs).
It would be apparent to one skilled in the art that in order to provide a full solar spectrum solar cell it should be structured so that the material at the front of the solar cell absorbs the shortest wavelengths and progressively longer wavelengths are absorbed by layers within the solar cell towards the lowermost surface. As such, the material within a full solar spectrum solar cell should grade from InxGa1-xN where x≈1 to InyGa1-yN where y≈0, i.e. be formed with InN at the substrate. Additionally the growth of InN onto compatible substrates, i.e. silicon, should be achieved relatively free of defects allowing not only the stress free growth of the necessary nanowire structures but also to facilitate the inclusion of multiple quantum wells, quantum dots and quantum-dots-within-quantum-dots which allow the efficiency of the solar cell to be improved.
Whilst the prior art includes growth of InN nanowires using foreign metal catalysts via the vapor-liquid-solid growth mechanism, see for example J. Li et al in U.S. Pat. No. 6,831,017 entitled “Catalyst Patterning for Nanowire Devices and C. Liang et al in “Selective-Area Growth of Indium Nitride Nanowires on Gold-Patterned Si(100) Substrates” (Appl. Phys. Lett., Vol. 81, 22 (2002), and spontaneous formation under nitrogen rich conditions; see for example C-K Chao et al “Catalyst Free Growth of Indium Nitride Nanorods by Chemical Beam Epitaxy” (Appl. Phys. Lett., Vol. 88) and S. Hersee et al in U.S. Pat. No. 7,521,274 entitled “Pulsed Growth of Catalyst-Free Growth of GaN Nanowires and Application in Group III Nitride Semiconductor Bulk Material”, each presents significant drawbacks for solar cells or solid state lighting applications including tapered morphology with large variations in the wire diameter along the wire length, substantial growth variations with compositional change, non-uniform nanowire length, as well as defects and stress which degrade quantum well and quantum dot structures.
Recent developments from the University of McGill however have demonstrated very high quality, uniform diameter and height nanowires of InN on silicon without foreign metal catalysts, see Y. Chang et al “Molecular Beam Epitaxial Growth and Characterization of Non-Tapered InN Nanowires on Si(111)” (Nanotechnology, Vol. 20, 2009) and Z. Mi et al in U.S. patent application Ser. No. 12/956,039 entitled “Method of Growing Uniform Semiconductor Nanowires without Foreign Metal Catalyst and Devices Thereof” thereby forming the basis for potential high efficiency, low cost, solar cells based upon these nanowires with graded composition, quantum wells and quantum dots. The growth technique by virtue of being applicable to group III nitrides with wurtzite structure was also used to grow nanowire GaN light sources with internal quantum efficiencies of 45% with unique quantum well and quantum-dot-within-a-quantum-dot structures, see Y. Chang et al in “High Efficiency Green, Yellow and Red Emission from InGaN/GaN Dot-in-a-Wire Heterostructures on Si(111)” and Z. Mi et al in US Patent Application “Method of Growing Uniform Semiconductor Nanowires without Foreign Metal Catalyst and Devices Thereof” and devices thereof” entitled “Method of Growing Uniform Semiconductor Nanowires without Foreign Metal Catalyst and Devices Thereof.” Such efficient green, yellow, and red emissions augmenting the existing GaN based blue LEDs.
Accordingly a “white” light source may be composed by assembling high efficiency blue, green, yellow, and red InGaN/GaN quantum-dot and nanowire based LEDs with suitable optical sub-assemblies to provide the necessary diffuse source without significant additional loss. Such assemblies whilst anticipated as commercially feasible require several LEDs to be manufactured on different silicon wafers, separated, assembled onto a carrier and electrically interconnected adding additional material costs and labor as well as increasing final “white” light source costs through yield reductions etc. Additional applications for high efficiency sources, which have not been reviewed in detail, include those within telecommunications at wavelengths such as 850 nm, 1300 nm, and 1550 nm in the near infrared.
Similarly a full spectrum solar cell exploiting high efficiency quantum dot and nanowire based p-i-n photodetectors would require assembly from multiple devices covering the near-ultraviolet (near-UV), blue, green, yellow, and red together with probably multiple devices covering the first near-infrared (near-IR) region of ˜750 nm to ˜1300 nm, and second near-IR region of ˜1500 nm to ˜1750 nm Again such multiple “colour banded” solar cells like “white” light sources requiring additional optical elements to split the incoming spectrum efficiently to each “colour banded” solar cell, multiple solar cells to be manufactured on different silicon wafers, separated, assembled onto a carrier and electrically interconnected adding additional material costs and labor as well as increasing final full spectrum solar cell costs through yield reductions etc. However, such an approach given the efficiencies of quantum dot and nanowire based solar cells are anticipated to be commercially feasible.
Colloidal Quantum Dots: It is within this context that semiconductor quantum dots, nanometer sized semiconductor particles which act as a very small “box” for electrons, and potentially the most efficient light sources offer a solution to reducing the number of discrete high efficiency LEDs/“colour banded” photodetectors required in white LED sources/full solar spectrum photodetectors and have thus formed the subject of significant research. Whilst one dimensional (1D) confinement of charge carriers in semiconductor quantum wells is now a well established method of enabling efficient optical gain and lasing, with improved performance metrics such as occupation thresholds, gain coefficients, differential gain, and temperature stability, it was predicted that three dimensional (3D) carrier confinement would increase the density of band-edge states relative to these 1D systems, further improving the performance of these materials as optical emitters or absorbers, see for example M. Asada et al (IEEE J. Quantum Electron., Vol. 22, 1986).
Strongly confined semiconductor quantum dots being particularly appealing, as 3D spherical confinement partitions the bulk electronic structure of the material into discrete transitions whose quantized energy levels are a pronounced function of particle size. Not only does this confinement allow for continuous tunability of the emission wavelength, but should also, in principle, result in reduced lasing thresholds with an associated enhancement of the differential gain which is particularly important for high efficiency optical sources, see V. I. Klimov (Semiconductor and Metal Nanocrystals: Synthesis and Electronic and Optical Properties, Published by Marcel Dekker, New York, 2004) and V. I. Klimov (Annu. Rev. Phys. Chem., Vol. 58, pp 635, 2007). Furthermore, it has been predicted that the occupation thresholds necessary to develop population inversions in these materials, as well as the differential gain in terms of state filling, should be entirely independent of particle size, see V. I. Klimov supra. As such, it was anticipated that strongly confined semiconductor quantum dots would be a universal, size tuneable, and highly efficient gain medium.
Much of the appeal of the colloidal quantum dot is that it can be readily integrated with other technology platforms at very low cost and that by varying the physical dimensions of the quantum dots they can be made to emit/absorb at points across the entire visible spectrum. Accordingly providing colloidal quantum dots with a range of dimensions within the same localized region acts to provide the required multiple sources to “blend” together to provide the illusion of a “white” light source or allows the same localized region to absorb photons over a wider wavelength range. Colloidal quantum dots are finding applications outside of photonics including for example their use in biological and chemical applications including providing markers and tags.
Limited Tunability: Recent work demonstrated the tunability of optical amplification and lasing using the size-dependent transition energies of strongly confined colloidal CdSe quantum dots, see for example V. I. Klimov et al (Science, Vol. 290, pp 314, 2000), Y. Chan et al (Appl. Phys. Lett., Vol. 85, 2004), and M. Caruge et al (Phys. Rev. B, Vol. 70, 2004). Unfortunately, these works have yet to realize the predicted size-universal development of optical gain in these systems, and are in general characterized by the need for specific host media, and progressively larger occupation thresholds as the particle radii are reduced. The difficulties arise due to the confinement enhanced interactions of the multiple excitations required to develop population inversions in the emitting transition, as well as the depletion of high energy charge carriers into surface or interface states, see for example R. R. Cooney et al in “Gain Control in Semiconductor Quantum Dots via State-Resolved Optical Pumping” (Phys. Rev. Lett., Vol. 102, 2009). In colloidal suspensions these impeding influences only allowed optical gain to be verified in relatively large particles.
A key result from the prior art is that multiexcitonic interactions related to quantum size effects may fundamentally impede the development of optical gain in strongly confined quantum dots. These confinement enhanced interactions result in a shift of the transition energies, often manifesting themselves as photonic absorption (PA) in the transient absorption (TA) spectra of these materials. In general, excited state charge distributions, in both the intrinsic quantized manifolds, as well as the extrinsic surface and interface states, are capable of producing this level shifting. Specific to CdSe quantum dots, excitations generally red shift the band-edge absorbing transition precisely into the region of the spontaneous photoluminescence (PL). Rather than generate optical gain under intense optical pumping, the complex interplay of the multiexcitonic interactions in the strongly confined quantum dot yields a PA at precisely the emitting wavelength. It is this confinement enhanced PA which is largely responsible for impeding the development of optical gain in these systems. The first demonstration of size-tunable optical gain in strongly confined semiconductor quantum dots was provided by V. I. Klimov (see supra) and illustrated that the development of optical gain in CdSe quantum dots was strongly dependent on the identity of their matrix material. Though the confinement based tunability of optical amplification was demonstrated, it could be achieved only under specific sample conditions.
In subsequent years, optical gain was observed in CdSe quantum dots in their native solution, but only for the largest particle sizes. It was argued that confinement enhanced multiexcitonic interactions competed with, and often completely overwhelmed, the development of optical gain for smaller particles. As the radii were reduced the observed occupancy threshold increased as a direct result of the increasing size dependent influence of the interfering PA. In hexane solution, optical gain could not be demonstrated in particles with radii smaller than 2.3 nm, thereby removing a significant portion of the spectral range arising from confinement based tunability. Furthermore, this work suggested potentially fundamental barriers related to quantum size effects. Similar results have been obtained in related materials such as CdS, PbS, PbSe, generally accompanied by even larger thresholds and smaller differential gains.
In addition to the multiexcitonic interactions, which were believed to result in gain blocking, it was believed that the gain lifetime in these quantum dots would be too short, due to enhanced Auger recombination rates. In order to bypass the perceived limitations of quantum dots, alternative materials such as quantum nanowires (rods or whiskers) were investigated. The underlying premise was that the nanowires may have more favourable gain characteristics due to weaker multiexciton interactions and/or slower Auger recombination times. In these systems there have been indications that the interfering multiexcitonic interactions and the development of optical gain were sensitive to the excitation wavelength.
Accordingly the last decade of prior art suggests that the pathway to a universal, size tunable nanocrystalline gain material lies either with development of new materials, see for example S. A. Ivanov et al (J. Phys. Chem. B, Vol. 108, 2004), S. Link et al (J. Appl. Phys., Vol. 92, 2002), M. Kazes et al (J. Phys. Chem. C, Vol. 111, 2007), H. Htoon et al (Appl. Phys. Lett., Vol. 82, 2003), V. I. Klimov et al (Nature, London, Vol. 447, 2007) and J. Nanda et al (J. Phys. Chem. C, Vol. 111, 2007) or with new host media, see for example V. I. Klimov et al (Science, Vol. 290, 2000), H-J. Eisler et al (Appl. Phys. Lett., Vol. 80, 2002) and Y. Chan et al (Appl. Phys. Lett., Vol. 85, 2004).
Universal Gain Behaviour: Accordingly, neither approach within the prior art of different materials or host media addresses the goal of providing quantum dots, either as discrete elements of, or as a part of either a light emitting source or a light absorbing detector that provide the required optical performance over a broad wavelength range by employing a wide distribution of particle dimensions. Even if the desired result is obtained in the future with new materials and new host media these must be compatible with semiconductor materials, semiconductor processing techniques, meet the environmental and performance requirements of the application over the intended wavelength range of the device. At present multiple materials and multiple host media are required to cover even the visible spectrum of 350 nm-750 nm without considering wider ranges such as the near-infrared or operation and tunability within the telecommunications windows between 1250 nm and 1650 nm.
It would therefore be beneficial to minimise the interference mechanisms that occur within smaller radii quantum dots such that optical emission is possible at all dimensions of quantum dot so that a single material/host can be employed in the applications discussed supra.